Microemulsions may be used for different chemical purposes. One well known use for oil-in-water microemulsions is to conduct surfactant floods to recover oil from underground formations. It is also believed that oil-in-water microemulsions generally perform better for hydrocarbon recovery when they are formulated with high alkane carbon number (ACN) oil. Such high ACN oils typically have an ACN 50% to 300% higher than the ACN of the crude oil for which the microemulsion is optimized.
The literature teaches the advantages of using low oil content microemulsions for enhanced oil recovery which are formulated with high ACN oils. U.S. Pat. No. 4,318,816 demonstrates that the stability of a single component surfactant such as an alkylarylpolyalkoxyalkylene sulfonate is enhanced when a high ACN oil is added to an aqueous surfactant to form an oil-in-water microemulsion. The high ACN oil raises the phase inversion temperature of the ethoxylated surfactant. U.S. Pat. No. 4,271,907 teaches that water soluble polymers are compatibly incorporated into an optimized microemulsion only when the microemulsion is formulated with a high ACN oil.
However, oil-in-water microemulsions are difficult to economically produce in the field on a field-wide scale. This difficulty is enhanced when high ACN oil is used to create the oil-in-water microemulsion. One known method of creating a water-in-oil emulsion with alkoxylated surfactants is to heat and then cool the mixture of surfactant, high ACN oil and brine. When heated, a water-in-oil microemulsion is formed because the alkoxylated surfactants become more hydrophobic as the temperature is increased. Upon cooling, the system is inverted to form a stable, translucent, oil-in-water microemulsion. This method is not practical in the field because large volumes of fluid must be heated to 20.degree. to 50.degree. C. above reservoir temperature.
A second method of producing stable oil-in-water microemulsions is to subject the mixture of surfactant, high ACN oil and brine to a very high shear. For example, a microemulsion having particle sizes less than 100 nm can be formed after multiple passes through a shear device having a pressure drop of about 14,000 psi. Since practical pressure drops obtainable in the field approach only about 2,000 psi, extensive shearing would be required to yield injectable oil-in-water microemulsions.